1. Field of the Invention
The present invention relates generally to Czochralski growth of crystalline ingots. In particular, the invention relates to a feed system for repetitively supplying feedstock for continuous Czochralski growth of crystalline ingots.
2. Description of the Related Art
Among the many types of photovoltaic solar cells being developed, one of the most efficient and economical cells is based on silicon wafers grown by the Czochralski method, similarly to that most prevalently used for silicon integrated circuits. In the Czochralski (CZ) method, silicon is melted in a crucible to its liquid state at a temperature of about 1420° C. A small crystalline silicon seed of predetermined crystalline orientation is contacted with the melt and then gradually withdrawn. With proper control of temperatures, the liquid silicon freezes on the crystalline seed with the same orientation as that of the seed. The seed is then slowly raised away from the melt to form a growing crystalline ingot of silicon having a final length typically of a meter or more and a diameter typically of hundreds of millimeters determined by the initial drawing conditions as the ingot expands from the diameter of the seed to the desired ingot diameter. In typical integrated applications, batch CZ is practiced in which the crucible is filled with an initial charge of electronic grade silicon, also called virgin poly or simply polysilicon. The crucible is then heated, one ingot is drawn to substantially deplete the crucible, and the crucible is discarded after one ingot. The cooled ingot is sliced to form circular monocrystalline wafers having a thickness of substantially less than a millimeter. See Wolf and Taber, Silicon Processing for the VLSI Era, vol. 1: Process Technology, Lattice Press, 1986, pp. 5-21, incorporated herein for reference, for discussions about electronic grade silicon and the typical Czochralski process.
Solar cell applications, however, are much more cost sensitive than silicon integrated circuits. In particular, the need to replace the crucible after each ingot imposes a high cost in crucibles and loss of the residual silicon left in the crucible. Also, the time required to replace the crucible and to reheat the crucible and its chamber greatly diminishes the throughput.
One general approach to reduce the cost of solar silicon involves continuous Czochralski (CCZ) growth, which has been known for many years but has not been widely practiced. It has recently been suggested for solar use. See U.S. Pat. No. 7,635,414 to Bender and U.S. Patent Application Publication No. 2011/0006240 to Williams et al. CCZ allows the drawing of multiple ingots from one crucible but requires the replenishment of fresh silicon into the generally small crucible while it is hot and most preferably while one of the ingots is being drawn from the crucible melt. However, the drawing of high-quality monocrystalline ingots requires tightly controlled temperatures, critical melt level control, and a non-reactive ambient such as argon. The replenishment of silicon feedstock to the Czochralski crucible is one of the challenges of CCZ.
Therefore, there is a need in the industry for a feed system capable of being used to continuously provide feedstock to a Czochralski growth system and, in particular, a continuous Czochralski growth system.